1. Field of the Invention
The present invention relates generally to a holdback device for rotary equipment and, more particularly, an apparatus which permits a preselected amount of additional rotation of a rotatable shaft in a reverse direction following the engagement of its holdback device for the purpose of permitting another rotatable shaft to rotate an additional amount to permit its holdback device to actuate.
2. Description of Prior Art
When more than one holdback device is utilized in association with a conveyor or similar apparatus, it is desirable that all of the holdback devices actuate when reverse movement of the conveyor device occurs. If the plurality of holdback devices are not synchronous and one actuates before the other holdback devices, it may stop the backward movement of the conveyor prior to the actuation of the other holdback devices and therefore must absorb all of the reverse forces being exerted by the conveyor system. In other words, the earliest holdback device to actuate when reverse movement of the conveyor occurs is forced to stop the movement of the entire conveyor system without help from other holdback devices because of the fact that the other holdback devices have not yet actuated and will not actuate since the conveyor is no longer moving backward as a result of the holding action provided from the earliest holdback device to actuate.
Many devices are known to those skilled in the art of mechanical equipment design that are provided for the purpose of stopping the rotation of one element relative to another element. For example, U.S. Pat. No. 4,697,672, which issued to Linton on Oct. 6, 1987, discloses a bi-directional force limiting no-back mechanism which includes both a rotatable and axially shiftable shaft with a plate secured thereto. Two ratchet wheels are coaxial with the shaft and relatively rotatable with respect to the shaft. The ratchet wheels sandwich a plate and have peripheral ratchet teeth. Pawls are engageable with the ratchet teeth on respective ones of the ratchet wheels and first and second sets of force limiting teeth are disposed on the sides of the ratchet wheels opposite the plate.
U.S. Pat. No. 2,372,026, which issued to Smith on Mar. 20, 1945, discloses a hoist load brake. It provides a roller ratchet mechanism that is suitable for use in a hoist load brake. Provision is made for automatically compelling a substantially uniform distribution of the braking load throughout a series of rollers so that individual rollers will not be damaged by having a disproportionate portion of a load inadvertently applied to them. It also provides a spring arrangement for yieldably biasing the ratchet rollers in such manner as to insure positive and rapid action without jamming by Permitting a wide latitude of tolerance in the dimensioning of the pockets which receive the rollers.
U.S. Pat. No. 3,695,402, issued to Klemm on Oct. 3, 1972, discloses a plunger stopped bi-directional no-back assembly that comprises a rotatable output member, a rotatable drive member for rotating the output member in either direction and a releasable brake for substantially preventing rotation of the output member in either direction when it is not being driven by the drive member. This device provides an apparatus on the drive member that is operable to release the brake upon a predetermined rotation of the drive member in either direction.
When a plurality of holdback devices, or no-back devices, are utilized in tandem to cooperate for the purpose of preventing reverse motion of a conveyor system or the like, non-synchronous actuation of the holdback devices creates a disadvantageous situation wherein one of the holdback devices must bear a disproportionate share of the load required to prevent further reverse operation of the conveyor apparatus. The earliest holdback device that actuates when reverse motion occurs acts to prevent further reverse motion. This stoppage of reverse motion by the earliest acting holdback device also prevents actuation of other holdback devices in most cases because those other holdback devices require a slightly additional reverse motion to cause them to actuate. Because of variations during their manufacture and as a normal result of machining tolerances, it is normal to expect one of the plurality of associated holdback devices to actuate before the others actuate. As a result, the earliest actuating holdback device is therefore forced to carry a large portion of the total load and, perhaps, the entire load if the other holdback devices are not actuated at all. A significant benefit could be realized if holdback devices were constructed in a manner that would permit all of the associated holdback devices to be actuated when the conveyor apparatus begins to move in a reverse direction. If all the associated holdback devices are actuated, they will all share the load and reduce the forces that would otherwise be experienced by a single holdback device that is forced to withstand the entire load of the conveyor system alone.
Applicants also filed another application, given Ser. No. 07/862,935, on the same day that this application was filed, assigned to the same assignee, also relating to holdback devices, but of different construction.